Find the 30-day cost of running a 100 W ceiling fan for 10 hours daily, using the textbook’s assumed price of Rs 25 per kWh.

Solution & explanation

1. Use the textbook’s hypothetical tariff, not a current electricity-price claim.

2. Convert watts to kilowatts.

   $$P=\frac{100}{1000}=0.1$$

3. Find total operating time.

   $$t=10\times 30=300$$

4. Energy in kWh equals kW multiplied by hours.

   $$E=0.1\times 300=30$$

5. Multiply units by price per unit.

   $$C=30\times 25$$

6. This excludes taxes and other charges.

   $$C=750$$

Energy = 30 kWh; assumed cost = Rs 750